West Nile Virus Facts: Preventing WNV in Horses

West
Nile Virus (WNV) is
a viral disease that can cause encephalitis or meningitis, infection of the
brain and the spinal cord or their protective covering. Prior to 1999, the
disease was found only in Africa, Asia, and southern Europe.
This virus has been identified in all of the continental United States, most of Canada and Mexico. The virus is transmitted
from avian reservoir hosts by mosquitoes (and infrequently by other
bloodsucking insects) to horses, humans and a number of other mammals.

West Nile virus (WNV) is the leading cause of arbovirus
encephalitis in horses and humans in the United States. Since 1999, over
25,000 cases of WNV encephalitis have been reported in U.S. horses.
Horses represent 96.9% of all reported non-human mammalian cases of WNV
disease.

The case fatality
rate for horses exhibiting clinical signs of WNV infection is approximately
33%.Data have supported that 40% of horses that survive the acute illness
caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities,
6-months post-diagnosis. Thus vaccination for West Nile virus is recommended as
a core vaccine and is an essential standard of care for all horses in North America.

Vaccinate annually in the spring, prior to the onset of the
insect vector season. For animals at high risk or with limited immunity, more
frequent vaccination or appropriately timed revaccination is recommended in
order to induce protective immunity during periods of likely exposure. For
instance, juvenile horses (<5 years of age) appear to be more susceptible
than adult horses that have likely been vaccinated and/or had subclinical
exposure. Geriatric horses (>15 years of age) have been demonstrated to have
enhanced susceptibility to WNV disease. Therefore, more frequent vaccination
may be recommended to meet the vaccination needs of these horses.

You
can reduce the number of mosquitoes around your home and neighborhood by
reducing the amount of standing water available for mosquito breeding. Here are
some simple steps you can take.

• Clean clogged roof gutters every year, particularly if
the leaves from surrounding trees have a tendency to plug up the drains.
Millions of mosquitoes can breed in roof gutters each season.

• Turn over plastic wading pools when not in use. A wading
pool becomes a place for mosquitoes to breed.

• Turn over wheelbarrows and don’t let water stagnate in
birdbaths. Both provide breeding habitats for domestic mosquitoes.

• Aerate ornamental pools or stock them with fish. Water
gardens can become major mosquito producers if they are allowed to stagnate.

• Clean and chlorinate swimming pools when not in use. A
swimming pool left untended by a family on vacation for a month can produce
enough mosquitoes to infest an entire neighborhood. Mosquitoes may even breed
in the water that collects on pool covers.

• Use landscaping to eliminate standing water that collects
on your property. Mosquitoes may breed in any puddle that lasts for more than
four days.

There
are also some easy steps you can take to prevent mosquitoes from affecting your
horses.

• House horses indoors during peak periods of mosquito activity
(dusk and dawn).

• Avoid turning on lights inside the stable during the evening
and overnight. Mosquitoes are attracted to lights.

• Place incandescent bulbs around the perimeter of the stable
to attract mosquitoes away from the horses. Black lights don’t attract
mosquitoes well.

• Remove all birds (including chickens) that are in or close
to the stable.

• Look around the property periodically for dead birds, such
as crows. Any dead birds should be reported to the local health department. Use
rubber gloves to handle dead birds or use an implement, such as a shovel.

• Topical preparations containing mosquito repellents are
available for horses. Read the product label before using and follow all
instructions.

• Use fans on the horses while in the stable to help deter
mosquitoes.

• Fog stable premises with a pesticide in the evening to reduce
mosquitoes. Read directions carefully before using.

For
help in assessing mosquito exposure risks on your property and for suggested control
practices, please contact your county
Extension office, county
Department of Environmental Protection, local Department of Health, local
veterinarian, or mosquito and pest control company.

A. Yes, while data suggest that most
horses infected with West Nile virus recover, results of investigations
indicate that West Nile virus has caused deaths in horses in the United States.

Q. How do the horses become infected
with West Nile virus?

A. The same way humans become
infected—by the bite of infectious mosquitoes. The virus is located in the
mosquito's salivary glands. When mosquitoes bite or "feed" on the
horse, the virus is injected into its blood system. The virus then multiplies
and may cause illness. The mosquitoes become infected when they feed on
infected birds or other animals.

Q. How does the virus cause severe
illness or death in horses?

A. Following transmission by an
infected mosquito, West Nile virus multiplies
in the horse's blood system, crosses the blood brain barrier, and infects the
brain. The virus interferes with normal central nervous system functioning and
causes inflammation of the brain.

Q. Can I get infected with West Nile virus by caring for an infected horse?

A. West Nile
virus is transmitted by infectious mosquitoes. There is no documented evidence
of person-to-person or animal-to-person transmission of West
Nile virus. Normal veterinary infection control precautions should
be followed when caring for a horse suspected to have this or any viral
infection.

Q. Can a horse infected with West Nile virus infect horses in neighboring stalls?

A. No. There is no documented evidence
that West Nile virus is transmitted between
horses. However, horses with suspected West Nile
virus should be isolated from mosquito bites, if at all possible.

A. No. EEE, WEE, and VEE belong to
another family of viruses for which there is no cross-protection.

Q. Can I vaccinate my horse against West Nile virus infection?

A. Yes, West Nile
virus vaccines for horses are available through veterinarians. Horse owners
throughout the US
should strongly consider vaccinating their equines. Consult your veterinarian
for more details on timing of vaccination.

Q. How long will a horse infected with West Nile virus be infectious?

A. We do not know if an infected horse
can be infectious (i.e., cause mosquitoes feeding on it to become infected).
However, previously published data suggest that the virus is detectable in the
blood for only a few days.

Q. What is the treatment for a horse
infected with West Nile virus?Should it be destroyed?

A. There is no reason to destroy a
horse just because it has been infected with West Nile
virus. Data suggest that most horses recover from the infection. Treatment
would be supportive and consistent with standard veterinary practices for
animals infected with a viral agent.